public class Solution {
    public int findMissingNum(int[] nums){
        int left = 0;
        int right = nums.length - 1;
        while(left < right){
            int mid = left + (right - left) / 2;
            if(nums[mid] > mid){
                right = mid;
            }else{
                left = mid + 1;
            }
        }
        return nums[left] == left ? left + 1 : left;
    }
}
